The invention relates notably to helium refrigerators/liquefiers generating very low temperatures (for example 4.5K in the case of helium) with a view to continuously cooling users such as superconducting cables or components of a plasma generation device (“TOKAMAK”). What is meant by a refrigeration/liquefaction device is notably the very low-temperature (cryogenic temperature) refrigeration devices and/or liquefaction devices that cool, and where appropriate liquefy, a gas with a low molar mass such as helium.
When the user is cooled down, which means to say when the user needs to be brought down from a relatively high starting temperature (for example 300K or above) to a determined low nominal operating temperature (for example around 80K). The refrigeration/liquefaction device is generally ill-suited to such cooling.
What happens, when heavy components (such as superconducting magnets for example) are cooled from ambient temperature down to 80K over a lengthy period (over a few tens of days), relatively hot and cold streams of helium (feed toward the user and return from the user) pass countercurrentwise through common exchangers. For the device to operate correctly though, it is necessary to limit the difference in temperature between these streams of helium (for example to a maximum difference of between 40K and 50K).
To do so, the device comprises an auxiliary pre-cooling system which supplies frigories during this cooling-down.
As illustrated notably in the article (“Solutions for liquid nitrogen pre-cooling in helium refrigeration cycles” by U. Wagner of CERN—2000), the pre-cooling system generally comprises a volume of liquid nitrogen (at constant temperature, for example 80K) which supplies frigories to the working gas via at least one heat exchanger.
These known pre-cooling systems do, however, have constraints or disadvantages.
Thus, it is necessary to mix helium at 80K with hotter helium (at ambient temperature or the temperature at which it returns from the user that is to be cooled).
In order to limit the consumption of liquid nitrogen it is moreover necessary to recover the frigories from the helium returning from the user that is to be cooled as the user is gradually cooled. These constraints on temperature difference and on performance require heat exchanger technologies that differ according to the various operating configurations (cooling-down, normal operation).
Thus, during normal operation (outside of the cooling-down phase), the exchangers need to have very high performance, i.e. low pressure drops and should not be faced with significant temperature differences. Heat exchangers suited to this normal operation comprise heat exchangers of the aluminum brazed plate and fin type. This type of exchanger can typically tolerate temperature differences of more than 50K between countercurrent fluids.
During the cooling-down of heavy users, the heat exchange performance required in the exchangers is not as high but remains high. By contrast, the temperature differences (because of the liquid nitrogen at constant temperature) become relatively great (greater than 50K).
When the helium temperatures in the circuits and exchangers are still high, the pressure drop is far greater than that required in normal operation.
Existing solutions for addressing these problems entail a main exchanger at the entrance to the cold box which provides an exchange of heat between the helium and the nitrogen. Other solutions make provision for this main exchanger to be split into several independent sections produced using different heat exchanger technologies according to the nature of the fluid (helium or nitrogen).
These solutions do not provide a satisfactory solution to the problems because the device is either ill-suited to normal operation or ill-suited to the cooling-down phase.